Fetal Hydronephrosis

Urinary tract dilation (UTD).

Fetal hydronephrosis (from Greek: ὕδωρ = water, νεφρός = kidney) describes a dilated fetal renal collecting system (1). This can refer to the dilation of either the fetal renal pelvis alone (pyelectasis or pelviectasis) or of both the fetal renal pelvis and the calices (pelvicaliectasis) (2). A subgroup of hydronephrosis, fetal pyelectasis is defined as an enlargement of the anterior-posterior renal pelvis diameter (1). Because clinicians tend to falsely equate the term hydronephrosis with obstruction many authors prefer the term urinary tract dilation (3).

Multiple classification systems have been used in the past for grading prenatal hydronephrosis. One of the most recent and generally accepted classification systems with adequate inter- and intra-observer reliability is the Urinary Tract Dilation (UTD) classification system. This system takes into consideration not only the anterior-posterior renal pelvis diameter (AP RPD), but also peripheral calyceal dilation, parenchymal thickness, parenchymal, ureter and bladder appearance and oligohydramnios from genitourinary cause (4, 5): 

O35.8XX0

Depending on the diagnostic criteria the prevalence of fetal hydronephrosis ranges from 0.6 to 5.4% (6). Antenatal hydronephrosis represents about 20% of all structural congenital abnormalities detected on prenatal ultrasound examinations (7). Male to female ratio is 2:1 (8). In the majority of cases the condition is unilateral (9).

• Transient or physiological hydronephrosis (50-70%)
• Ureteropelvic Junction Obstruction (UPJO) (10-30%)
• Vesicoureteral Reflux (VUR) (10-40%)
• Ureterovesical junction obstruction/megaureter (5-15%)
• Multicystic kidney disease (MCDK) (2-5%)
• Posterior urethral valves (PUV) (1-5%)
• Other causes, (Ureterocele, sacrococcygeal teratoma, prune-belly syndrome, retrocaval ureter, ectopic ureter, urethral atresia, hydrometrocolpos, cloacal abnormalities, polycystic kidney disease) (3)

Mild fetal hydronephrosis has been reported as a soft marker for Trisomy 21, which is why this finding should prompt a detailed assessment of fetal anatomy to exclude other anomalies (10). However one should keep in mind that fetuses with isolated fetal hydronephrosis appear not to be at increased risk for Down's syndrome if they had an earlier screening which was found to be reassuring (11).

Establishing the diagnosis

• Image both kidneys. If the renal pelves appear enlarged, a measurement should be documented (12).
• Measure the maximum AP RPD in the transverse plane with spine at 12 or 6 o'clock if feasible (3). Do not forget that maternal hydration status, the degree of bladder distension, or recent urination may affect the degree of dilatation (8).
• Evaluate the renal parenchyma appearance (normal: iso- or hypoechoic to liver, without cysts) (3).
• Evaluate of the renal parenchyma thickness, which can be measured in the sagittal plane at the level of the mid-kidney as the shortest distance from the renal sinus to the inner aspect of the renal capsule, where ≥7mm is generally considered normal in the third trimester (13).
• Evaluate the presence of peripheral calyceal distention, which is abnormal.
• Evaluate the ureter. Do not forget that the fetal ureter may sometimes be visible as a thin structure in its anatomic location as urine is being passed from the kidney towards the bladder. Transverse diameter ≥ 3mm is considered to be abnormal (14).
• Evaluate the bladder, note a possible dilation, wall thickness, or a keyhole sign, suggestive of lower urinary tract obstruction (LUTO).
• Evaluate the amniotic fluid volume.

First Trimester Diagnosis

The diagnosis of fetal hydronephrosis can be made as early as the first trimester, since the fetal kidneys can be visualized most of the times (80-86% at 11 weeks, 86-97% at 12 weeks, 92-99% at 13 weeks) (15, 16). For the pathological AP RPD in the first trimester a threshold of 1.5mm has been suggested (17).

3D Imaging

Use of the three-dimensional renal parenchyma volume / kidney volume ratio in the third trimester has been suggested in an effort to enhance our ability to estimate the prognosis (18).

While establishing the definite diagnosis isn't prenatally always possible (19), below are some diagnostic clues that can lead towards a possible diagnosis.

• If the renal pelvis and calyceal dilation end abruptly at the level of the ureteropelvic junction ureteropelvic junction obstruction (UPJO) should be suspected.
• Depicting an enlarged bladder with the keyhole sign and bilateral hydronephrosis with oligohydramnios is suggestive of lower urinary tract obstruction (LUTO).
• An obstructed upper pole and ureterocele are pathognomonic for renal duplication.
• A distended ureter without ureterocele or signs of renal duplication suggests ureterovesical junction obstruction.
• A variable hydronephrosis, which increases after voiding could be a sign for vesicoureteral reflux (VUR).
• A male fetus with cryptorchidism, deficient abdominal wall muscles, and dilation of bladder, ureters and kidneys are typical findings of the prune-belly syndrome (20).

Since antenatal hydronephrosis represents about 20% of all structural congenital abnormalities detected on prenatal ultrasound examinations (7), and considering the severity of long-term complications like recurrent urinary tract infections or renal insufficiency in the pediatric population, sonographic screening appears reasonable. Both kidneys should be imaged. If the renal pelves appear enlarged, a measurement should be documented (12). The maximum AP RPD in the transverse plane ideally with the fetal spine at 12 or 6 o'clock should be measured (3). Pathological results should lead to targeted examination by trained specialists.

After diagnosing antenatal hydronephrosis the clinician’s aim should be to provide accurate information to the physicians taking care of the infant postnatally and to counsel the parents. The sonographic findings should be described in accordance with the UTD classification system and representative images should be included. When there is an antenatal suspicion for potential need for surgical intervention or risk for renal compromise, consultation prior to delivery with a pediatric urologist and/or pediatric nephrologist should be undertaken to help outline the care that the child may require postnatally. Also, if not previously done, the maternal risk for aneuploidy should be assessed (3).

While most cases are transient, or physiological, the likelihood of a significant postnatal renal abnormality is proportional to the severity of the fetal hydronephrosis. About 5% of patients with fetal hydronephrosis will ultimately require surgery (19).

In a prospective study 12% of fetuses identified using a 4-mm threshold for AP RPD during the second trimester and whose AP RPD remained <7mm in the third trimester showed a significant urologic abnormality (8). In addition hydronephrosis has been show to recur after antenatal resolution had been documented (19).

Antenatal Follow-Up

In case of UTD A1 an additional ultrasound at ≥32 weeks is recommended. Cases of UTD A2-3 should be followed-up initially in 4-6 weeks; certain situations (e.g. bilateral hydronephrosis, suspicion for LUTO) require more expedient follow-up or even consideration of drainage procedures (3).

Obstetric Management

No modification of standard obstetric care is required (7).

Fetal Surgery

In cases of fetal lower urinary tract obstruction (LUTO) the chance of newborn babies surviving with normal renal function is very low irrespective of whether or not vesicoamniotic shunting is done (21).

Postnatal Follow-Up

In case of UTD A1 the first ultrasound examination of the newborn should be at least >48 hours after delivery up to 1 month later, while even a normal finding should prompt a second ultrasound examination 1-6 months later. If  the hydronephrosis was classified antenatally as UTD A2-3, the first ultrasound would still be performed at least >48 hours after delivery up to 1 month later, although certain situations may require earlier intervention (3).

1. ^{Capolicchio JP, Braga LH, Szymanski KM. Canadian Urological Association/Pediatric Urologists of Canada guideline on the investigation and management of antenatally detected hydronephrosis. Can Urol Assoc J. 2018;12(4):85–92. doi:10.5489/cuaj.5094}
2. ^{Chiodini, B., Ghassemi, M., Khelif, K., & Ismaili, K. (2019). Clinical Outcome of Children With Antenatally Diagnosed Hydronephrosis. Frontiers in Pediatrics, 7, 103. https://doi.org/10.3389/fped.2019.00103}
3. ^{Nguyen, H. T., Benson, C. B., Bromley, B., Campbell, J. B., Chow, J., Coleman, B., … Stein, D. R. (2014). Multidisciplinary consensus on the classification of prenatal and postnatal urinary tract dilation (UTD classification system). Journal of Pediatric Urology, 10(6), 982–998.doi:10.1016/j.jpurol.2014.10.002}
4. ^{Hodhod, A., Capolicchio, J.-P., Jednak, R., El-Sherif, E., El-Doray, A. E.-A., & El-Sherbiny, M. (2016). Evaluation of Urinary Tract Dilation Classification System for Grading Postnatal Hydronephrosis. The Journal of Urology, 195(3), 725–730.doi:10.1016/j.juro.2015.10.089}
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7. ^{Benjamin, T., Amodeo, R. R., Patil, A. S., & Robinson, B. K. (2016). The Impact of Gestational Age at Delivery on Urologic Outcomes for the Fetus with Hydronephrosis. Fetal and Pediatric Pathology, 35(6), 359–368. https://doi.org/10.1080/15513815.2016.1202361}
8. ^{Ismaili, K., Hall, M., Donner, C., Thomas, D., Vermeylen, D., & Avni, F. E. (2003). Results of systematic screening for minor degrees of fetal renal pelvis dilatation in an unselected population. American Journal of Obstetrics & Gynecology, 188(1), 242–246. https://doi.org/10.1067/mob.2003.81}
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